The American Lung Association (ALA) estimates that nearly 16 million Americans suffer from chronic obstructive pulmonary disease (COPD) which includes diseases such as chronic bronchitis, emphysema, and some types of asthma. The ALA estimated that COPD was the fourth-ranking cause of death in the U.S. The ALA estimates that about 14 million and 2 million Americans suffer from emphysema and chronic bronchitis respectively.
Those inflicted with COPD face disabilities due to the limited pulmonary functions. Usually, individuals afflicted by COPD also face loss in muscle strength and an inability to perform common daily activities. Often, those patients desiring treatment for COPD seek a physician at a point where the disease is advanced. Since the damage to the lungs is irreversible, there is little hope of recovery. Most times, the physician cannot reverse the effects of the disease but can only offer treatment and advice to halt the progression of the disease.
To understand the detrimental effects of COPD, the workings of the lungs requires a cursory discussion. The primary function of the lungs is to permit the exchange of two gasses by removing carbon dioxide from arterial blood and replacing it with oxygen. Thus, to facilitate this exchange, the lungs provide a blood gas interface. The oxygen and carbon dioxide move between the gas (air) and blood by diffusion. This diffusion is possible since the blood is delivered to one side of the blood-gas interface via small blood vessels (capillaries). The capillaries are wrapped around numerous air sacs called alveoli which function as the blood-gas interface. A typical human lung contains about 300 million alveoli.
The air is brought to the other side of this blood-gas interface by a natural respiratory airway, hereafter referred to as a natural airway or airway, consisting of branching tubes which become narrower, shorter, and more numerous as they penetrate deeper into the lung. Specifically, the airway begins with the trachea which branches into the left and right bronchi which divide into lobar, then segmental bronchi. Ultimately, the branching continues down to the terminal bronchioles which lead to the alveoli. Plates of cartilage may be found as part of the walls throughout most of the airway from the trachea to the bronchi. The cartilage plates become less prevalent as the airways branch. Eventually, in the last generations of the bronchi, the cartilage plates are found only at the branching points. The bronchi and bronchioles may be distinguished as the bronchi lie proximal to the last plate of cartilage found along the airway, while the bronchiole lies distal to the last plate of cartilage. The bronchioles are the smallest airways that do not contain alveoli. The function of the bronchi and bronchioles is to provide conducting airways that lead air to and from the gas-blood interface. However, these conducting airways do not take part in gas exchange because they do not contain alveoli. Rather, the gas exchange takes place in the alveoli which are found in the distal most end of the airways.
The mechanics of breathing include the lungs, the rib cage, the diaphragm and abdominal wall. During inspiration, inspiratory muscles contract increasing the volume of the chest cavity. As a result of the expansion of the chest cavity, the pleural pressure, the pressure within the chest cavity, becomes sub-atmospheric. Consequently, air flows into the lungs and the lungs expand. During unforced expiration, the inspiratory muscles relax and the lungs begin to recoil and reduce in size. The lungs recoil because they contain elastic fibers that allow for expansion, as the lungs inflate, and relaxation, as the lungs deflate, with each breath. This characteristic is called elastic recoil. The recoil of the lungs causes alveolar pressure to exceed atmospheric pressure causing air to flow out of the lungs and deflate the lungs. If the lungs' ability to recoil is damaged, the lungs cannot contract and reduce in size from their inflated state. As a result, the lungs cannot evacuate all of the inspired air.
In addition to elastic recoil, the lung's elastic fibers also assist in keeping small airways open during the exhalation cycle. This effect is also known as “tethering” of the airways. Tethering is desirable since small airways do not contain cartilage that would otherwise provide structural rigidity for these airways. Without tethering, and in the absence of structural rigidity, the small airways collapse during exhalation and prevent air from exiting thereby trapping air within the lung.
Emphysema is characterized by irreversible biochemical destruction of the alveolar walls that contain the elastic fibers, called elastin, described above. The destruction of the alveolar walls results in a dual problem of reduction of elastic recoil and the loss of tethering of the airways. Unfortunately for the individual suffering from emphysema, these two problems combine to result in extreme hyperinflation (air trapping) of the lung and an inability of the person to exhale. In this situation, the individual will be debilitated since the lungs are unable to perform gas exchange at a satisfactory rate.
One further aspect of alveolar wall destruction is that the airflow between neighboring air sacs, known as collateral ventilation or collateral air flow, is markedly increased as when compared to a healthy lung. While alveolar wall destruction decreases resistance to collateral ventilation, the resulting increased collateral ventilation does not benefit the individual since air is still unable to flow into and out of the lungs. Hence, because this trapped air is rich in CO2, it is of little or no benefit to the individual.
Chronic bronchitis is characterized by excessive mucus production in the bronchial tree. Usually there is a general increase in bulk (hypertrophy) of the large bronchi and chronic inflammatory changes in the small airways. Excessive amounts of mucus are found in the airways and semisolid plugs of this mucus may occlude some small bronchi. Also, the small airways are usually narrowed and show inflammatory changes.
Currently, although there is no cure for COPD, treatment includes bronchodilator drugs, and lung reduction surgery. The bronchodilator drugs relax and widen the air passages thereby reducing the residual volume and increasing gas flow permitting more oxygen to enter the lungs. Yet, bronchodilator drugs are only effective for a short period of time and require repeated application. Moreover, the bronchodilator drugs are only effective in a certain percentage of the population of those diagnosed with COPD. In some cases, patients suffering from COPD are given supplemental oxygen to assist in breathing. Unfortunately, aside from the impracticalities of needing to maintain and transport a source of oxygen for everyday activities, the oxygen is only partially functional and does not eliminate the effects of the COPD. Moreover, patients requiring a supplemental source of oxygen are usually never able to return to functioning without the oxygen.
Lung volume reduction surgery is a procedure which removes portions of the lung that are over-inflated. The portion of the lung that remains has relatively better elastic recoil, providing reduced airway obstruction. The reduced lung volume also improves the efficiency of the respiratory muscles. However, lung reduction surgery is an extremely traumatic procedure which involves opening the chest and thoracic cavity to remove a portion of the lung. As such, the procedure involves an extended recovery period. Hence, the long term benefits of this surgery are still being evaluated. In any case, it is thought that lung reduction surgery is sought in those cases of emphysema where only a portion of the lung is emphysematous as opposed to the case where the entire lung is emphysematous. In cases where the lung is only partially emphysematous, removal of a portion of emphysematous lung which was compressing healthier portions of the lung allows the healthier portions to expand, increasing the overall efficiency of the lung. If the entire lung is emphysematous, however, removal of a portion of the lung removes gas exchanging alveolar surfaces, reducing the overall efficiency of the lung. Lung volume reduction surgery is thus not a practical solution for treatment of emphysema where the entire lung is diseased. Moreover, conventional lung volume reduction surgery is an open surgical procedure which carries the risk of surgical complications and requires a significant period of time for recuperation.
Both bronchodilator drugs and lung reduction surgery fail to capitalize on the increased collateral ventilation taking place in the diseased lung. There remains a need for a medical procedure that can alleviate some of the problems caused by COPD. There is also a need for a medical procedure that alleviates some of the problems caused by COPD irrespective of whether a portion of the lung, or the entire lung is emphysematous. The production and maintenance of collateral openings through an airway wall allows air to pass directly out of the lung tissue responsible for gas exchange. These collateral openings serve to decompress hyperinflated lungs and/or facilitate an exchange of oxygen into the blood.
Methods and devices for creating and maintaining collateral channels are discussed in U.S. patent application Ser. No. 09/633,651, filed on Aug. 7, 2000; U.S. patent application Ser. Nos. 09/947,144, 09/946,706, and 09/947,126 all filed on Sep. 4, 2001; U.S. Provisional Application No. 60/317,338 filed on Sep. 4, 2001; U.S. Provisional Application No. 60/334,642 filed on Nov. 29, 2001; U.S. Provisional Application No. 60/367,436 filed on Mar. 20, 2002; and U.S. Provisional Application No. 60/374,022 filed on Apr. 19, 2002 each of which is incorporated by reference herein in its entirety.
Although creating an opening through an airway wall may overcome the shortcomings associated with bronchodilator drugs and lung volume reduction surgery, various problems can still arise. When a hole is surgically created in tissue the healing cascade is triggered. This process is characterized by an orderly sequence of events, which can be broadly classified into distinct phases. These phases proceed in a systematic fashion, with a high degree of integration, organization, and control. However, the various stages are not sharply delineated, but overlap considerably, and factors affecting one phase have a stimulatory or inhibitory effect on the overall process.
The result of this wound healing process is tissue proliferation that can occlude or otherwise close the surgically created opening. Additionally, in the event an implant is deployed in the surgically created opening to maintain the patency of the opening, the implant may become encapsulated or filled with tissue thereby occluding the channel.
Drug eluting coronary-type stents are not known to overcome the above mentioned events because these stents are often substantially cylindrical (or otherwise have a shape that conforms to the shape of a tubular blood vessel). Hence, they may slide and eject from surgically created openings in an airway wall leading to rapid closure of any channel. Additionally, the design and structure of the coronary-type stents reflect the fact that these stents operate in an environment that contains different tissues when compared to the airways not to mention an environment where there is a constant flow of blood against the stent. Moreover, the design of coronary stents also acknowledges the need to place the stent within a tubular vessel and avoid partial restenosis of the vessel after stent placement so that blood may continue to flow. In view of the above, implants suited for placement in the coronary are often designed to account for factors that may be insignificant when considering a device for the airways.
Not surprisingly, experiments in animal models found that placement of coronary drug eluting stents (i.e., paclitaxel drug eluting vascular stents and sirolimus drug eluting stents) into the airway openings did not yield positive results in maintaining the patency of the opening. The shortcomings were both in the physical structure of the stent which did not lend itself to the airways as well as the inability of those drug eluting devices to control the healing cascade caused by creation of the channel. The majority of these devices filled with tissue at an early stage and an inspection of the remainder of the implanted devices indicated imminent closure.
An understanding of the distinctions between the healing response in the coronary versus the airways may explain this outcome. For purposes of our discussion, the healing response in both the coronary and the lungs may be divided into approximately four stages as measured relative to the time of the injury: 1) acute phase; 2) sub-chronic phase; 3) chronic phase; and 4) late phase.
In the coronary, after trauma caused by the placement of a coronary stent, the healing process begins in the acute phase with thrombus and acute inflammation. During the sub-chronic phase, there is an organization of the thrombus, an acute/chronic inflammation and early neointima hyperplasia. In the following chronic phase, there is a proliferation of smooth muscle cells along with chronic inflammation and adventitial thickening. In the late stage of the healing process there is chronic inflammation, neointimal remodeling, medial hypertrophy and adventitial thickening.
Based upon the observations in a rabbit model, the healing response in the airway begins with a fibrinous clot, edema hemorrhage, and fibrin deposition. In the sub-chronic phase there is re-epithelialization, mucosal hypertrophy, squamous metaplasia, fibroplasias and fibrosis. In the chronic phase, while the epithelium is intact and there is less mucosal hypertrophy, there is still fibroplasia and fibrosis. In the late stage the respiratory epithelium is intact and there is evidence of a scar.
Accordingly, the unique requirements of the airways and collateral channels calls for specific features for any implant used in collateral channels. For example, these implants/conduits are often placed across three different tissue zones; namely the parenchyma, the newly sectioned airway wall, and the interior of the airway surface. Each different zone may have a different reaction to the presence of the implant/conduit. The parenchyma may build up a layer of scar tissue around the conduit, which may eventually eject the implant or block the air path on the parenchyma side of the conduit. The airway wall may undergo a healing response as a result of the trauma of the procedure. This healing response and associated tissue growth may restrict air-flow through the implant. Furthermore, mucus from the airways may deposit in to the conduit thereby further occluding the conduit.
In addition, placement of an implant or conduit within the collateral channel may present additional structure requirements for the devices. For example, surgeons often use radiological imaging to place coronary stents within the vasculature. In most cases, placement of coronary stents is critical so that the ends of the coronary stent straddle the vascular obstruction. In contrast, a surgeon placing an implant in collateral channels is often using a remote access device such as a bronchoscope or endoscope that allows for direct observation of the device during placement. For proper placement of the implant, and in cases where it is important to “sandwich” the airway wall, it is necessary to identify the center and/or edges of the conduit or implant prior to expansion of the device. It follows that failure to properly place the implant may result in detachment of the implant (via insufficient attachment to the airway wall), pneumothorax (if the implant is advanced too distally and breaches the pleural cavity), or deployment of the implant wholly in the lung parenchyma exterior to the airway wall. Accordingly, such devices may require a visual indicator to assist the medical practitioner during placement and to offer a measure of safety so that the device is not improperly advanced/deployed thus creating additional complications.
Accordingly, there remains a need for devices and methods that specifically address the requirements discussed herein.